[听力原文]11-15Venice is a fascinating city between sea and sky. It is built on 117 islands. There aren’t any cars and buses, But it has 150 canals and 400 bridges. The narrow streets, with their historic names, are paved with stones but have no footpaths. They are lined with flower, shops and lights side by side. The squares are charming, The brick bridges are built high to; allow boats to pass tinder them.Every year thousands of tourists front the five continents visit this city. They are always amazed at the charm of her water and beautiful lights, which can make them free from all dust and cooled by the sea breezes.But because the level of the surrounding waters is constantly rising, the Venice faces a threat to its Very existence. And the Venetian love their city and want to stay there to save the Venice from the sea, Now various methods hove already been taken and a plan to protect the city is under investigation. Now what steps have already been taken to protect the city
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A.IL-1 B.IL-2C.IL-3 D.EPOE.IFN-α 促进红细胞生成的是()
A. IL-1
B. IL-2
C. IL-3
D. EPO
E. IFN-α
某地为了了解新研制的流感气雾免疫制剂的效果,在本地随机确定5000人接种疫苗,另外5000人不接种该疫苗作为对照,经过一个流行期后,前者50人发病,后者500人发病,则 该疫苗的保护率为
A. 10%
B. 10
C. 2.7%
D. 90%
E. 以上都不对
用于确证人类致癌物的研究方法是
A. 构效关系分析
B.致突变试验,恶性转化试验
C.哺乳动物短期致癌试验
D.哺乳动物长期致癌试验
E.流行病学调查
An airplane flies because its wings push down on the air flowing past them, and in reaction, the air pushes up on the wings. When an airplane is level or rising, the front edges of its wings ride higher than the rear edges. The angle the wings make with the horizontal is called the angle of attack. As the wings move through the air, this angle causes them to push air flowing under them downward. A sleeper angle of attack will cause the wings to push more air downward. The third law of motion voiced by English physicist Isaac Newton states that every action produces an equal and opposite reaction. In this case, the wings pushing air downward is the action, and the air pushing the wings upward is the reaction. This causes lift, the upward force on the plane, Lift is often explained using Bernoulli’s principle, which states that, in some circumstances, a faster moving fluid(流体) (such as air)will have a lower pressure than a slower moving fluid. The air on the top of an airplane wing is at a lower pressure than the air underneath the wing, but this is an effect of lift, not its cause.Lift is one of the four primary forces acting upon an airplane. The others are weight, thrust, and drag. Weight is the force that offsets lilt. because it acts in the opposite direction. The weight of the airplane must be overcome by the lift produced by the wings, if an airplane weighs 4.5 metric tons, then the lift produced by its wings must be greater than 4.5 metric tons in order for the airplane to leave the ground. Designing a wing that is powerful enough to lift an airplane off the ground, and yet efficient enough to fly at high speeds over extremely long distances, is one of the marvels of aircraft technology. What does the article tell us aboutWhy does an airplane fly